The present invention relates to implantable prosthetic valves and stents for use therein.
A prosthetic valve for use in replacing a natural valve of the cardiovascular system may incorporate a stent that acts as a supporting structure and a plurality of leaflets typically formed from a thin flexible material. The leaflets function in a manner similar to the leaflets of a natural valve so as to allow blood to flow in one direction but block blood flow in the opposite direction. For example, such a valve may be implanted at or near the site of the natural aortic valve to replace a defective aortic valve. The leaflets open during systole to allow flow in the distal direction from the left ventricle of the heart into the aorta, but close during diastole to block reverse blood flow in the proximal direction from the aorta into the heart.
Historically, prosthetic aortic valves have been implanted in an open-chest surgical procedure. More recently, valves suitable for implantation using a catheterization technique have been proposed. Such valves typically incorporate a stent generally in the form of an elongated. The valve leaflets are mounted to the stent. Typically, a covering of a flexible material commonly referred to as a cuff overlies a portion of the stent. The entire assembly, including the stent, leaflets, and cuff, is collapsible in radial directions relative to the axis of the stent. When the assembly is in a fully collapsed condition, the assembly is small enough in diameter to be carried on a catheter through the vascular system to the site of implantation. For example, a catheter may be inserted into an access site, such as the femoral artery, and advanced through the descending aorta, around the aortic arch, and into the ascending aorta to the site of the native aortic valve. At the implantation site, the stent is expanded to an expanded condition in which the stent grips the surrounding tissue and secures the assembly in place. In the expanded condition, the valve flaps are free to open and close to provide an action similar to that of the native valve. Expansion of the stent may be driven by an expansion device such as a balloon carried on the catheter inside the stent. In other cases, the stent is a self-expanding structure, typically formed from a metallic material such as a nickel-iron memory alloy commonly referred to as “nitinol” or other metallic alloy. Expandable stents of this type typically are formed as a mesh-like structure incorporating numerous elongated members joined at their ends with one another, so as to define cells of the mesh. For example, the members may be formed integrally with one another by etching or cutting a thin tube of the stent material so as to leave the mesh-like structure.
Collapsible and expandable valves of this nature most commonly are made with the flaps formed from a biologically derived material as, for example, bovine or porcine pericardial tissue. The cuffs typically are made from natural tissues or from synthetic material such a Dacron™ polyester fabric. It has also been proposed to form the valve leaflets from synthetic polymeric materials such as polyurethane compositions. U.S. Pat. Nos. 6,953,332 and 7,682,389, the disclosures of which are incorporated by reference herein, disclose certain valves having leaflets formed from synthetic polymers. Co-pending, commonly assigned U.S. Provisional Patent Application No. 61/512,999, filed Jul. 29, 2011, the disclosure of which is also incorporated by reference herein, discloses improvements in processes for forming valve leaflets by a dip-molding process, as well as an improved mandrel for use in such processes.
The leaflets of a prosthetic valve are subjected to significant stresses. Typically, each leaflet is a thin sheet having a curved free edge and another curved edge attached to the stent. The leaflets come together with one another at points referred to as commissures positioned at the ends of the attached edge. When the valve is in a closed condition, the free edges of the leaflets are bowed into the center of the stent so that the leaflets abut one another along their free edges or “coapt” with one another, so as to form a closed surface spanning the interior of the stent. When the leaflets are in the open position, the free edges of the leaflets are displaced outwardly towards the periphery of the stent, so that the free edges of the leaflets cooperatively define an opening for flow of the fluid. In the closed condition, the pressure of the blood distal from the valve is greater than the pressure of the blood proximal from the valve. In this condition, the pressure distal to the valve tends to pull the attached edges of the leaflets away from the stent and thus puts stress on the attachments between the attached edges and the stent. This stress is applied repeatedly, with every beat of the heart. As a replacement valve typically is expected to last for many years, it must withstand many billions of cycles of such stress.
Valve flaps formed from natural tissues typically are sutured to the stent. For example, as disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 13/237,237, the disclosure of which is incorporated by reference herein, the attached edges of the stent may extend along some of the elongated members of the stent and may be sutured to such members.
Despite considerable attention devoted in the art heretofore to development of implantable valves and stents for the same, still improvement would be desirable. In particular, it would be desirable to provide a valve with improved attachment between the flaps and the stent, and to provide a stent having attachment features particularly suitable for forming such secure attachment. It would also be desirable to provide a valve incorporating a stent with such improved attachment. Such improvements would be desirable in all valves, and would be particularly desirable in the case of valves having leaflets formed from synthetic polymeric materials.